The present invention relates generally to a voltage clamp circuit used in a switching circuit which switches inductive loads and, more particularly, to an improved active clamp circuit. The improved active clamp circuit includes a zener diode and permits independent control of the operating speed of the switching circuit and zener breakdown current to prevent generation of excessive levels of an electrical disturbance referred to as microplasmic noise (MPN).
When the current in an inductive load is switched off, voltage peaks or overshoot pulses, often referred to as "flyback voltage", are generated because of the voltage-current relationship for inductive loads. This flyback voltage can lead to destruction of the device which switches current to the load and also generates substantial noise which can be coupled into and interfere with operation of associated or nearby circuitry. Fortunately, flyback voltage can be dissipated or discharged in a controlled manner in one of a number of known arrangements including clamp diodes and active clamp circuits to essentially prevent generation of interfering noise levels.
In one active clamp circuit of the prior art, a conventional diode and a zener diode are connected in series, either cathode-to-cathode or anode-to-anode, across the collector-base or drain-gate of a power transistor used to switch or control an inductive load. In this circuit, the flyback voltage is advantageously clamped without an additional power device. Current due to the flyback voltage is conducted through the conventional and zener diodes to partially activate the power transistor which then provides a controlled current path through which the inductive load is discharged.
It is also often desirable to slow down the switching circuit to reduce noise which would otherwise be produced by switching on the inductive load since such noise can also interfere with operation of associated or nearby circuitry the same as noise generated by the flyback voltage. Circuit slow down is typically accomplished by increasing the resistance of a base or gate resistor of a power transistor used to switch the inductive load. Unfortunately, in the prior art active clamp circuit, the resistor also determines the zener breakdown current. Accordingly, increased resistance not only slows circuit operation but also reduces and limits the zener breakdown current.
The zener breakdown current can be reduced to current levels which are in the "knee" region of the reverse IV characteristic curve of the zener diode. When zener diodes are operated at such low currents, they produce a significant level of electrical disturbance called microplasmic noise (MPN). MPN is oscillatory noise which is the result of the zener breakdown phenomenon and is generally considered "white" noise with equal amplitude for all frequencies from about zero hertz to approximately 200 kilohertz. MPN levels decrease as zener current increases and also change with the temperature of the zener diode junction, generally increasing for low junction temperatures. MPN is sometimes specified as a zener noise density by zener diode makers, see for example the Motorola Rectifiers and Zener Diodes Data Book published by Motorola, Inc.
If MPN is generated by a zener diode used in an active clamp for a switching circuit, the MPN is coupled to the base or gate of the power transistor and amplified. The active clamp provides a closed loop path which feeds the amplified noise back to the base or gate of the power transistor and a resulting low-Q, high current oscillator circuit operates momentarily during the clamping operation.
Clamped switching circuits are often closely associated with other circuits. For example, clamped switching circuits may be included in an automotive electronic circuit module used to control an antilock braking system or other system of an automobile. Thus, switching noise including MPN in an active clamp circuit can not only adversely affect the operation of the switching circuits themselves but also be coupled into other associated circuitry and adversely affect their operation as well.
A possible solution to the MPN problem is to select a zener diode based upon the zener breakdown current which is generated in the switching circuit. However, such selection would restrict the number of available devices to those which meet the zener breakdown current specifications and are properly specified by the manufacturer. Further, it is likely that any diode so selected would be a more expensive device than otherwise required due to the more stringent specifications.
Thus, there is a need for an active clamp for a switching circuit which switches inductive loads wherein an inexpensive zener diode can be used consistently without the generation of MPN by the zener diode. It would be desirable to be able to control the operating speed of the switching circuit and also the zener breakdown current substantially independently of one another. Independent control would optimize the operation of the switching circuit, and ensure that the associated active clamp will operate properly to limit flyback voltage without generating interfering levels of MPN.